In known suspension systems resilient means such as springs or torsion bars are provided to perform a multiplicity of functions ranging from the absorption of impact loading (as from hitting bumps at speed) to the provision of flexible support to enable all the wheels to maintain ground contact when traversing uneven terrain. Additionally applied loads such as cargo deflects traditional suspensions to induce movement between the body and wheels in a similar manner to dynamic or contour loadings.
Traditionally resilient sprung suspensions are based on each wheel assembly being provided with an individual resilient component which mechanically supports the respective "corners" of the vehicle. The resilient components have rapidly progressive load rating as deflected and normal vehicle weight is only distributed to all wheels when wheels collectively describe a flat plane surface. When one wheel of a vehicle passes over (or is parked on) a bump, that wheel carries more vehicular weight than it normally carries on flat ground. Meanwhile the adjacent wheels are correspondingly relieved of some of their normal share of the weight.
The rapidly progressive resilient sprung suspension systems work satisfactorily only within a narrow spectrum of dynamic, static and applied loading situations, and any type of overloading or even underloading of a vehicle normally adversely affects its abilities to maintain traction, average ground clearance, and quality of ride. Moreover the scope of demands upon known resilient suspension systems lead to self conflicting performance characteristics as there is no inherent ability in the system to detect and react differently to diverse situations, which cause resonant rebounding, requiring excessive damping with shock absorbers, and also anti-roll bars, thus limiting free movement of unsprung components.
Recently there has been a trend towards resilient sprung suspension systems incorporating variable damping and spring rates in an attempt to redress some of the above referred to shortcomings. Some other more advanced suspension systems (active and semi-active suspensions) incorporate a number of electronic sensors which monitor information such as vertical wheel travel and body roll, as well as speed, acceleration, steering and braking commands. This and other data is processed by a computer which instructs hydraulic or pneumatic actuators to override the normal function of resilient springs in order to interpret, compensate and adjust the suspensions performance to suit speed, terrain and other factors in order to maintain a level ride and even distribution of weight onto wheels. These suspension systems therefore require an external intelligent back-up system, and call for substantial input of external energy, drawn from the vehicle engine, to operate the actuators that affect the adjustment to the suspension system. A range of constructions of `active` and `semi-active` suspensions for vehicles have been proposed including systems operating on the basis of compression and/or displacement of fluids and such systems currently in use incorporate a pump to maintain the working fluid at the required pressure and effect the high speed distribution thereof, and sophisticated control mechanisms to regulate the operation of the suspension system in accordance with sensed road and/or vehicle operating conditions. These known systems incorporating pumps and electronic control systems, are comparatively expensive to construct and maintain, and require energy input, and therefore have limited acceptability in the vehicle industry.
There has been proposed, such as in U.S. Pat. No. 4,606,551, damping systems used in conjunction with conventional sprung suspensions wherein fluid damping devices associated with individual wheels are interconnected to provide additional damping action during lateral rolling or longitudinal pitching movements. Although these constructions may contribute to improve damping performance the desirable characteristics of the basic sprung suspension, of rapidly progressive change in spring forces, lead to undesirable changes in weight distribution and limited wheel movement are still present.